In the latest device adopting a silicon wafer having a large diameter, for example, typified by a diameter of 300 mm, a size of a surface waviness component that is called nano-topography has been a problem in recent years. The nano-topography is a kind of a surface shape of a wafer and indicates a concave-convex shape of a wavelength component having a wavelength of 0.2 to 20 mm that is shorter than Sori or Warp and longer than surface roughness. The nano-topography is a very shallow waviness component having a PV value of 0.1 μm to 0.2 μm. It is said that this nano-topography affects a yield of an STI (Shallow Trench Isolation) process in a device manufacturing process. A strict level is required of the nano-topography for the silicon wafer to be a device substrate together with more minute design rules.
The nano-topography is made in a step of processing of the silicon wafer. It is easy to deteriorate particularly in a processing method with no reference plane such as wire saw slicing or double-disc grinding, and it is important to improve and to manage a relative wire meandering in the wire saw slicing and a Warp of the wafer in the double-disc grinding.
Here, a double-disc grinding method using a conventional double-disc grinding apparatus will be explained.
FIG. 4 are schematic views showing an example of a conventional double-disc grinding apparatus.
As shown in FIG. 4(A), a double-disc grinding apparatus 101 comprises a rotatable holder 102 for supporting a sheet-like wafer 103 from an outer circumference side along a radial direction, a pair of static pressure supporting members 112 for supporting the holder 102 from both sides of the holder along an axis direction of rotation without contact by static pressure of a fluid, the static pressure supporting members which are located at both sides of the holder 102 respectively, and a pair of grindstones 104 for simultaneously grinding both surfaces of the wafer 103 supported with the holder 102. The grindstones 104 are attached to a motor 111 and can rotate at a high speed.
As shown in FIG. 4(B), the holder 102 is provided with a protruding portion 105, and the protruding portion is engaged with a notch portion 106, such as a notch indicating a crystal orientation of the wafer and the like, formed on the wafer 103. Such double-disc grinding apparatus 101 that the wafer is ground with the protruding portion 105 of the holder 102 being engaged with the notch portion 106 of the wafer 103 is disclosed, for example, in Japanese Unexamined Patent publication (Kokai) No. 10-328988.
When both surfaces of the wafer 103 are ground by using the double-disc grinding apparatus 101, first, an outer circumference portion of the wafer 103 is supported by the holder 102 with the notch 106 of the wafer 103 being engaged with the protruding portion 105 of the holder 102. It is to be noted that the wafer 103 can rotate by the rotation of the holder 102.
A fluid is supplied between the holder 102 and each static pressure supporting member 112 from each of the static pressure supporting members 112 located at both sides, and the holder 102 is supported along an axis direction of the rotation by static pressure of the fluid. In this way, the wafer 103 is supported by the holder 102 and the static pressure supporting members 112 to rotate, and the both surfaces of the wafer 103 are ground by using the grindstones 104 caused to rotate at a high speed with the motor 111.